The fish of the paper nautilus is a type of octopus belonging to the genus ARGONAUTA. They live in the open ocean near the surface. The female argonaut is unique among modern cephalopods (squids, cuttlefish, octopus and nautilus) in producing a shell that serves as a brood chamber. She lives unattached within the shell, gripping onto it by holding her arms in a reflexed position with suckers adhering to the shell opening. The shell is also held in place by broad webs extending from the dorsal, uppermost pair of arms. These highly extensible webs, which may cover the entire shell, secrete the calcium carbonate and organic material from which the shell is made. Eleanor Alliston, in her book "Escape to an Island" describes watching one make a new shell. Adult females can be 10-15 times larger than adult males, which rarely exceed two centimetres in total length.

The eggs, when fertilised, are laid and incubated within the shell, where the female can conveniently oxygenate and clean them. Large numbers of eggs are laid in branching strings probably over several nights. A single egg mass carried by a female thus contains eggs at various stages of development. Upon hatching, argonaut young, as with other cephalopods, are extremely precocious and almost immediately begin jet- swimming and hunting small crustaceans. The newly hatched young are about 1.5mm long.